fix: export editable local dependencies with relative paths

[tunglambk]

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

Fixes #3294.

Editable local dependencies are now exported using their relative project path instead of expanding file:///${PROJECT_ROOT} into an absolute file:// URL. This keeps generated requirements files portable across machines and checkout locations.

The change is limited to editable FileRequirement entries that have a local path. VCS, remote URLs, and non-editable requirements keep their existing formatting.

A regression test covers a monorepo-style ./packages/pkg-core editable dependency and verifies that the exported line stays relative.

Verification

• Regression test added for the reported case.
• Local test execution was not available in the current environment; CI is expected to validate the full suite.

[frostming]

CI is failing

[tunglambk]

Thanks for the review. I checked the CI failure and confirmed it comes from the regression test I added (Namespace(..., self=False, ...), which conflicts with Namespace.__init__). I’m going to close this PR for now rather than leave it in a broken state. Sorry for the noise, and thanks for taking the time to review it.